Heat a wok until very hot. Add the vegetable oil and heat until shimmering. Add the garlic and stir-fry for 10 seconds. Add the shrimp and a pinch of salt and stir-fry until the shrimp are just beginning to turn pink, about 30 seconds. Add the green beans and 2 tablespoons of the stock and stir-fry until the beans soften slightly, about 3 minutes. Add the remaining 2 tablespoons of stock along with the fish sauce and a generous pinch of pepper and stir-fry until the shrimp are cooked through and the beans are crisp-tender, about 3 minutes longer. Serve right away.